1. Field of the Invention
This invention relates to the field of electric logging of xe2x80x9cwellsxe2x80x9d or earthen boreholes. In particular, the invention relates to well logging apparatus and methods for determining formation properties, such as resistivity, at several different distances extending radially from the borehole into the surrounding formation. The invention has general applications in the well logging art, but is particularly useful in measurement while drilling (xe2x80x9cMWDxe2x80x9d) applications.
2. Description of the Related Art
Resistivity logging is a commonly used technique for evaluating potential hydrocarbon-bearing formations surrounding a borehole drilled into the earth. Porous formations are more resistive to a flow of electric current when they are saturated with hydrocarbons, and less resistive when saturated with water (which contains some amount of salt, rendering it more or less conductive). The formation immediately surrounding the borehole can be altered by invasion of borehole fluids during the drilling of the well, and can therefore exhibit a different resistivity than the formation farther from the boreholexe2x80x94so-called xe2x80x9cvirginxe2x80x9d formation. In order to determine the true resistivity of the virgin zone, the well logging device must be capable of performing measurements at multiple depths of investigation. The multiple depths permit mathematical correction of the different measured values.
Historically, resistivity logging tools, conveyed by wireline after the borehole has been drilled, have measured resistivity at three depths of investigation (shallow, medium, and deep). Mathematically, the three measurements are used to solve for three unknowns (Rt, Rxo, and Di). The shallow and medium measurements are used to correct the deep measurements to obtain a more accurate measurement of true virgin resistivity (Rt). The medium and deep readings are used to correct the shallow reading to obtain a more accurate reading of flushed zone resistivity (Rxo) (the flushed zone being the formation nearest the borehole, in which the original formation fluids have been at least partially displaced by drilling fluids). The three readings are also used to determine the depth of invasion Di (that is, how far drilling fluids have intruded into the formation), when a simple step invasion profile is assumed.
Large values for depth of invasion indicate zones of high permeability, which suggest potential high fluid flow rates, desirable for producing commercially significant quantities of hydrocarbons. Computed values for Rt and Rxo may be used for estimating water saturation (Sw), under certain favorable conditions. Low values of Sw indicate the presence of hydrocarbons in the formation.
The present invention relates to a type of resistivity well logging known as electromagnetic propagation logging. Propagation logging is well suited for determining resistivity by apparatus designed for use while drilling, so-called MWD tools. The basic principle of such measurement is a transmitter propagating electromagnetic energy into the formation, at a known frequency and strength, and reflections of that transmitted energy are detected by receivers spaced apart from the transmitter. Earlier generation MWD propagation resistivity devices provided only two depths of investigation, from the phase difference and attenuation measurements. By xe2x80x9cphase differencexe2x80x9d is meant a difference in timing between the transmitted and received signal. By xe2x80x9cattenuationxe2x80x9d is meant a lessening or decrease in the amplitude of the transmitted signal.
Separation of the curves is used to identify invasion; however, it is mathematically impossible to solve for the three desired unknowns (Rt, Rxo, Di) from only two measurements. Another disadvantage of the earlier generation tools is that the vertical response of the attenuation measurement is not as sharp as the vertical response of the phase difference measurement. As a result, separation of the curves results at bed boundaries (that is, the boundaries between beds of dissimilar rock type within a zone), even when invasion is not present. Also, it is known in the art that dielectric uncertainty can cause the phase difference and attenuation curves to separate even when no fluid invasion is present. In fact, the separation of phase difference and attenuation curves can be used to estimate the dielectric constant in thick beds.
Another disadvantage of the attenuation measurement is reduced dynamic range when compared to the phase difference measurement. As the formation resistivity increases, the attenuation measured between the two receiver antennas approaches a constant value, and the measurement becomes insensitive to changes in resistivity. In contrast, the phase difference measurement retains sensitivity to higher resistivity values and thus has a broader useful range. The limited dynamic range of the attenuation measurement sets an upper resistivity limit on the utility of apparatus employing this method for detecting invasion.
More recent propagation MWD resistivity devices have added measurements at additional depths of investigation. However, these prior art apparatus and method still have various limitations. One group of apparatus achieves the multiple depth resistivity measurements via additional transmitter and receiver antennas, each tuned to transmit or receive at the same frequency but spaced differently, thereby resulting in different depths of investigation. The additional transmitters and receivers, it will be appreciated, added greatly to cost and complexity of the tools.
Yet another group of apparatus employed multiple different frequencies to yield multiple depths of investigation (it being known in the art that different frequencies yield different depths of investigation, the lower frequencies yielding a deeper investigation, while higher frequencies yield a shallower depth of investigation). However, this group of tools still required multiple additional transmitters and receivers, each tuned to transmit or receive only a single frequency. Again, increased cost and complexity of tools resulted. Many of these prior art apparatus exhibit other limitations, such as high electrical power consumption.
The apparatus and method of the present invention provide resistivity measurements at multiple (three or more) depths of investigation while avoiding the disadvantages of related art methods and apparatus. The apparatus and method herein provide multiple resistivity measurements that have nearly equivalent vertical resolution and maximum dynamic range, by using only phase difference measurements. Since attenuation measurements are not used for additional depths of investigation, the attenuation measurements can be combined with the phase difference measurements to solve for the formation dielectric constant at multiple frequencies. The current apparatus minimizes the number of antennas required for either a borehole compensated and electronically compensated measurement (four), or alternatively for an uncompensated measurement (three), as a result minimizing manufacturing and maintenance cost and maximizing reliability. Furthermore, untuned coils are used for the transmitter and receiver antennas, allowing each coil to be used for more than one frequency and eliminating error caused by mutual inductance between adjacent series tuned receiver antennas. The apparatus minimizes electronics required to transmit multiple frequencies by using a switch-mode transmitter circuit, which has the further advantage of generating the desired frequencies simultaneously. The transmitter electronics disclosed are also simpler and more efficient than methods used in the prior art. Transmitter energy is minimized by using low noise electronics and coherent detection in the receiver. Time required to complete a measurement can be minimized by simultaneously detecting multiple frequencies in the receiver.
Accordingly one of the objects of this invention is to provide resistivity measurements of a formation surrounding a borehole at multiple (three or more) depths of investigation into the formation. The advantage of this invention is that the additional measurements can be used to compute true or virgin formation resistivity, flushed zone resistivity, depth of investigation, and additional parameters useful in evaluating the economic potential of an oil or gas well.
Another object is to provide measurements at multiple depths of investigation with nearly equivalent vertical response and maximum dynamic range. The advantage is that differences of the measurements caused by mismatched vertical resolution and limited dynamic range are minimized, further minimizing false indications of invasion and error in determination of true formation resistivity.
Another object is to provide an estimate of formation dielectric constant by using the attenuation measurement in combination with the phase difference measurement at each frequency to determine both formation resistivity and dielectric constant. The advantage is that more accurate corrections for variations in dielectric constant can be applied to the resistivity data.
Another object is to minimize the number of transmitting and receiving antennas required for measurements at multiple depths of investigation. The advantage is that the device will have lower manufacturing and operating cost and greater reliability than prior art devices.
Another object is to use untuned coils for both the receiver and transmitter antennas. The advantage is that untuned coils are less expensive to build and maintain and are more reliable. A further advantage is that untuned coils can be used to transmit or receive multiple frequencies, unlike prior art devices, which require separate coils for each measurement frequency. Another advantage is that untuned receiver antennas do not suffer from errors caused by circulating currents in series-tuned receiver antennas.
Another object is to minimize the complexity and maximize the efficiency of electronics used to drive the transmitter antennas at multiple frequencies. The advantage is that manufacturing and maintenance costs will be reduced, reliability increased, and power consumption minimized, increasing battery life or allowing a smaller power source to be used.
Another object is to transmit and detect multiple frequencies simultaneously. The advantage is that the total time required for measurements of multiple depths of investigation is minimized compared to prior art devices and methods.
Another object is to minimize the required transmitter energy and maximize the dynamic range of the measurements by using low noise electronics and coherent detection. The advantage is that the reduced transmitter energy results in longer battery life or allows smaller power sources to be used, without compromising the accuracy and resolution of the measurements.
Further objects and advantages will become apparent from consideration of the drawings and ensuing description thereof.
The present invention is directed to a well logging apparatus having features that are responsive to a number of needs of the prior art, as discussed above. Most of the features of the invention as set forth herein generally have application to both wireline logging and measurement while drilling. However, some of the features hereof are particularly advantageous for use in a measuring while drilling apparatus.
In accordance with a feature of the invention, there is provided an apparatus and method for investigating earth formations in which resistivity is determined at three or more depths of investigation radially into the formation while using signals transmitted from either a single transmitter antenna or a pair of transmitter antennas placed symmetrically around a pair of receiving antennas. In an embodiment of this form of the invention using a single transmitter antenna, electromagnetic energy is transmitted at a first location in the borehole (the active transmitter antenna) and received at a second and third location (the receiving antenna pair). In an embodiment with two transmitter antennas, electromagnetic energy is also transmitted from a fourth location in the borehole and received at the second and third locations, following transmission from the first location. The measurements of the received signals from both transmissions are optionally combined to cancel errors resulting from the borehole or imbalance in the electronics.
The electromagnetic energy is transmitted at a fundamental frequency and also at harmonics of the fundamental frequency. The receivers at the second and third location determine the phase difference between the two receiving locations, at each frequency of interest. The formation resistivity at the deepest depth of investigation is determined from the phase difference measurement of the fundamental (lowest) frequency. The resistivity of the formation closer to the borehole is determined from the phase difference of the higher frequency harmonics. As the frequency increases, the measurement distance away from the borehole decreases. By using multiple frequencies, with each antenna capable of transmitting and receiving multiple frequencies, the total number of antennas required to obtain measurements at multiple depths of investigation is minimized.
The attenuation of the electromagnetic energy between the second and third location at each frequency is also determined, and is combined with the phase difference measurements at each frequency to simultaneously determine the formation dielectric constant as well as a dielectric-corrected resistivity. The vertical response of the phase difference measurements are well matched, so that they may be easily combined to evaluate the invasion profile of borehole fluids into the formation and determine true resistivity, flushed zone resistivity, and depth of invasion. Also, the phase difference measurements have a wider usable dynamic range than the attenuation measurements.
In accordance with another feature of the invention, there is provided an apparatus and method wherein all of the antennas are simple, untuned coils of wire. The elimination of tuning allows each antenna to be used at multiple frequencies, eliminating the need for individually tuned antenna coils for each frequency. Minimizing the number of antenna coils lowers manufacturing and maintenance cost, and improves reliability. The elimination of tuning also eliminates mutual coupling of magnetic fields in the closely spaced receiver coils due to circulating currents in low impedance series tuned antenna coils.
In accordance with another feature of the invention, there is provided an apparatus and method wherein the electronics used to drive the transmitter antennas consist of a simple switchmode amplifier topology driven by a derivative of the system clock. The pulsating waveform used to drive the transmitter antenna contains energy at the fundamental frequency and also at higher harmonics of the fundamental frequency. The pulsating waveform provides a simple, convenient method for generating the additional frequencies of interest. Switchmode operation of the electronics delivers high efficiency with minimal power dissipation and self heating in the electronics components.
In accordance with another feature of the invention, there is provided an apparatus and method wherein the receiver is capable of detecting multiple frequencies simultaneously. The signals received by the receiver antennas and amplified with a low noise RF amplifier are digitized directly and processed to extract the phase and amplitude information at each frequency of interest. The RF amplifier includes filtering to limit the bandwidth of the signal to be digitized, in order to prevent aliasing of noise into the measurement. The anti-aliasing filters insure good performance in conditions of low received signal to noise ratio, and allow less energy to be used to drive the transmitter. Detecting multiple frequencies simultaneously results in the shortest possible measurement time.
In accordance with another feature of the invention, there is provided an apparatus and method wherein transmitter energy is minimized by using low noise receiver electronics and coherent detection.